In photodiodes (APD's), avalanche photocurrent is multiplied by impact ionisation of primary photogenerated carriers accelerated by a high field. In III-V APD devices for use at wavelengths in the regions of 1.3 and 1.5 microns a material such as indium gallium arsenide or suitable quanternary compounds may be used for the region where photogeneration occurs. If the multiplication also takes place in this material, which has a comparatively small band gap, high dark currents resulting from tunnelling occur at the high fields required for multiplication, giving rise to excessive noise. To avoid this problem the carrier pairs are photogenerated in indium gallium arsenide and then the holes are swept into a wider band gap indium phosphide layer containing the pn junction where avalanche multiplication takes place. This type of device is known as the separate absorption and multiplication (SAM) structure. A further modification is the SAGM which has a graded layer of intermediate composition (usually a quaternary) between the indium phosphide and indium gallium arsenide layers. The thickness and doping level of the indium phosphide multiplication layer has to be carefully controlled in order to achieve the correct field for avalanche multiplication near the pn junction, with a sufficiently low field at the interface with the indium gallium arsenide layer to avoid tunnelling. Both planar and mesa APD structures are known, and in general planar structures have proved superior, and are therefore preferred.
In planar APD devices the pn junction, which is generally formed by p+ implantation or diffusion into a nominally n type layer, will have a curved edge and is therefore subject to edge beakdown unless the boundary shape is modified to minimise the effects. Thus it is usual for a p type guard ring, also diffused or implanted, to be fabricated around the outer edges of the p type junction region. The structure and doping concentrations within the guard ring influence the extent to which edge breakdown is prevented.